This is a slight modification to jonaskuehling's extruder, such that a GRRF hotend can be mounted on it. Tested with an actual GRRF hotend.Print using 0.25mm layer thickness (otherwise the bridges are not sliced correctly), drill through the bridge layers, attach your hotend with two M3x30 screws and nuts, print an idler and gears from http://www.thingiverse.com/thing:18379 , assemble and extrude away!